ANNIHILATION OF INCOMPLETELY THERMALIZED POSITRONIUM STUDIED BY AGE-MOMENTUM CORRELATION

The dependence of the Doppler broadening of the 511 keV photon annihil ation line of positrons annihilating in a variety of solid and Liquids is studied as a function of the positron age (= time spent by the pos itrons in the sample between injection and annihilation) by age-moment um correlation (AMOC) measurements. At small positron ages an extra br oadening of the momentum distribution of the annihilating positron-ele ctron pairs is observed in all positronium-forming materials investiga ted (fused quartz, boron nitride, methanol, n-hexane) but not in mater ials in which there is no evidence for positronium formation (Al, diam ond, Al2O3). The ''young age broadening'' is attributed to delayed the rmalization of positronium atoms with kinetic energies of a few electr on volts.